1. Field of the Invention
The present invention relates to a system employing an infrared camera for monitoring an abnormal condition of facilities. More particularly, this invention relates to a monitoring system which can avoid a malfunction caused by a reflection of sun light, etc. when the reflection is within the scene to monitor.
2. Description of the Related Art
The monitoring system has been widely employed for monitoring, for example, an outdoor transformer station where many of large electric apparatus, such as, transformers, circuit breakers, are situated. If some part of these apparatus becomes abnormally hot due to some reason, this fact must be urgently detected so as to take a proper action. Therefore, an infrared camera is provided to constantly monitor the apparatus so that the temperature rise at the monitored apparatus caused from something abnormal can be urgently recognized by a person in charge of the monitor. Therefore, it is required for the monitoring system to accurately operate achieving low erroneous detection rate.
FIG. 1 schematically shows a block diagram of a prior art system disclosed in Japanese Unexamined Patent Publication Tokukai HEI-1-288086, which is also now pending in U.S. patent application No. 07/726,669. FIG. 2 shows a flow chart of the image processing in the FIG. 1 system. In the FIG. 1 system, the temperature data output from an infrared camera 1 is converted to digital data, which is then alternately stored in frame memories 3 and 4 according to a control of a write controller 2 (step 50 in FIG. 2). Next, for each of the pixels, the previously stored temperature data is subtracted from the last stored temperature data in a differential operator 5 (step 52). Prior to the differential operation, an offset-adding is operated so that the last stored temperature data becomes always higher than background data in the previously stored data (i.e. the data before the temperature rise takes place); accordingly, the results of the differential operation should always become positive (step 51). This is because, without the offset-adding operation, the result of the differential operation may become either positive or negative to cause a complicated differential operation. The output of differential operator 5 is input to a TV monitor 6, where the temperature rise data is displayed as an image, as well as sent to a binarization circuit 7, where only the area of the temperature-rise is obtained (step 53). That is, when the operation result exhibits the same value as the offset-added value the pixel is recognized to be in the background area (having no temperature rise); and when the operation result exhibits other values than the offset-added value the pixel is recognized to be in a temperature rising area. The output of binarization circuit 7 is input to a histogram operation circuit 8, where the temperature rise data is processed to make a histogram of pixel quantities grouped in predetermined temperature ranges (step 54). When the pixel quantities in particularly predetermined temperature ranges are more than a predetermined level, it is recognized that an abnormal state has taken place (step 55); then an alarm device 9 raises an alarm.
In the above monitoring system, a monitored object, for example a transformer installed in an outdoor transformer station, may be lighted by the sun to cause a bright reflection therefrom, which then may be input into the infrared camera to cause a problem. That is, if the temperature to be detected by the monitoring system is in the range of several tens of degrees centigrade to several hundreds of degrees centigrade and the reflecting light is also in the range of several tens of degrees centigrade to several hundreds of degrees centigrade, the reflection may cause the system to erroneously detect an erroneous temperature rise of the transformer. Similar problems may arise when the sun lights an automobile situated aside the transformer, and the reflection therefrom is input to the infrared camera. In the latter case, there is also another problem in that avoiding the reflection from the automobile to the camera may reduce the monitoring field of vision of the camera.